714
Anal. Chem. 1988, 60,714-719
Glycerol- Induced Reduction in Electrohydrodynamic Mass Spectrometry J o h n H. Callahan,’ Kevin Ho01,~J o h n n y D. Reynolds, a n d Kelsey D. Cook* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600
Proton nuclear magnetic resonance spectroscopy Is used to quantitatively confirm the extent of reduction of the diquaternary ammonium salt m-phenylenebis(trimethyiammonium iodide) observed by electrohydrodynamk mass spectrometry (EHMS). The t h e scale over which reduction occurs Is on the order of hours, so the reaction can be followed directly in the mass spectrometer. Acceleration of the rate of reduction by I-Is observed. The quantitative glycerol-induced reduction of the Ru( III)-tris(Mpyridyi) complex observed by EHMS is confkmed by ultraviolet-visible spectroscopy. EHMS studies of Co( III),Cu( II),NI( II),and Cr( III)salts and comp(8xes provide evidence that glycerol can reduce spedes with E o greater than about 0.1 V (vs NHE). Thls Is consistent with resuits of recent fast atom bombardment (FAB) studies, suggesting that 801118 of the reduction observed In FAB occurs in solution prior to sampling.
particle bombardment. Furthermore, reliance on field evaporation of ions results in deposition of little excess energy, minimizing fragmentation and facilitating studies of solution chemistry (15-19). However, accurate quantitation of solution behavior by EHMS requires an understanding of the factors affecting the sampling of ions from solution. Sampling efficiencies depend on several factors, including analyte-solvent interactions, desolvation after sampling, ion pairing, and mass transport processes (19-21). Despite these complications, EH mass spectra have been found to reflect the solution chemistry of glycerol (19). For example, evidence for the following reaction was found in the EH mass spectrum of a glycerol solution of m-phenylenebis(trimethy1ammonium iodide) (15): QkCH313 *N(CH3),
m/z = “Soft” masa spectrometric ionization techniques that sample
ions directly from solution (1-5) have introduced the possibility of using mass spectrometry to probe solution chemistry, as well as to quantitate mixtures in solution. However, the invasiveness of various ionizing probes must be considered in connection with quantitative or even qualitative interpretation of “matrix-assisted” mass spectra. An important example arises from evidence for redox reactions with the fast atom bombardment (FAB) matrix, as noted by several investigators (6-11). For example, DePauw and co-workers (6) observed one-electron reduction of several metal ions and metal ion complexes and reduction by hydrogen transfer in a series of quinoidal dyes. For both inorganic and organic ions, the relative intensities of oxidized and reduced species observed in FAB spectra qualitatively followed the expected redox behavior (based on standard electrode potentials). Similar correlation was reported by Gale et al. (12) in liquid secondary ion (L-SIMS) experiments. These investigators established (by comparing L-SIMS and fission fragment spectra) that the solvent participates in reduction reactions. Cerny and Gross (13)proposed that this participation involves multiple protonation followed by electron transfer during desorption. Other investigators (6, 7, 11) have further suggested that this reduction is due to bombardment-induced formation of hydrogen radicals and electrons (11,14). The fact that reduction does not occur as readily in other solvents (e.g. tetraglyme and diethanolamine) (6) suggests that glycerol may play a specific chemical role in the reduction process, independent of an invasive primary beam. Isolation of such a role would help clarify the extent of bombardment-induced perturbation of solution equilibria. Electrohydrodynamic (EH) mass spectrometry provides an opportunity to study solutions without complications from Current address: Naval Research Laboratory, Washington, DC 20375-5000. Current address: Department of Chemistry, University of Illinois, Urbana, IL 61801.
-
+ Hk(CH31,
@kCH313 H
io6
m/z
145
m/Z = 69
Deuteriation of the methyl groups of the diquat (I) and subsequent EHMS analysis showed that hydrogen transfer to the aromatic ring system involved the solvent. The absence of reduction in aqueous solution (corroborated by EHMS and NMR studies of aqueous I (22)) indicates that in this case glycerol acted as a chemical reducing agent in solution. This raises the possibility that some of the redox chemistry observed elsewhere in FAB studies may actually occur prior to bombardment. We now report further EHMS and corroborative evidence of the activity of glycerol as a reducing agent in solution. While the results presented do not refute the possibility that bombardment-induced reduction is an important process in FAB, they emphasize the importance of considering the chemical properties of the solvents used when interpreting mass spectral results. EXPERIMENTAL SECTION Mass spectra were obtained with a double-focusing mass spectrometer (AEI MS-902) equipped with a VG Analytical electronics console (MS-9 update). The design and operation of the EH source have been described elsewhere (1, 23). Spectra were obtained at emitter potentials of 7-8 kV. The emitter voltage was usually optimized to match the ESA voltage (avoiding detection of “fast metastable” ions (20) experiencing in-flight desolvation prior to the ESA) by maximizing the signal intensity of the ion at m/z 207 ([Na + G$, where G denotes solvating glycerol). In cases where scans of “fast metastable” ions were obtained, the ESA voltage was lowered to allow passage of ions with less than full acceleration energy. In all MS studies, the extractor voltage was approximately -1.5 kV, and the collector was at ground potential. Concentrations for mass spectral samples are reported as mole percent (relative to glycerol = 100 mol 70).Solutions were degassed for at least 8 h at low heat (50 “C) under Torr). In time-dependence studies, solutions vacuum (
0003-2700/88/0360-07 14$01.5i/O 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
containing only glycerol and supporting electrolyte were degassed, then the analyte was added immediately prior to analysis. Steady emission could be maintained for periods of up to an hour, after which it became necessary to supply more solution to the emitter tip by advancing the plunger on the syringe containing the sample. Spectra were obtained with a 200 pm diameter platinum capillary (Hamilton). Results reported represent the average of at least three mass spectra. Prior to averaging, spectra were normalized (generally, most intense ion = 100%) to reduce deviations caused by fluctuating emission intensity. When an internal standard was used, spectra were normalized to the intensity of the internal standard peak (typically [K Gz]+and/or [K + G3]+). Proton NMR spectra were obtained with a Nicolet NT-200 spectrometer. A 5-100 mg portion of analyte was dissolved per gram of glycerol, with low heating for about 10 min. Stirring was continued (with no additional heating) for a given time period. Subsequently, 20-100 mg of the glycerol solution was diluted in 1-2 g of DzO, quenching the reaction and giving an ultimate analyte concentration of 0.05-10 mg/g of D,O. (CH3)3SiCHzCHz0S03Na(DSS) was used as an internal reference. UV-vis spectra were obtained with a GCA-McPherson Model EU-707 or a Varian/Cary 219 double beam spectrophotometer. Spectra of aqueous and glycerol solutions were obtained with the corresponding pure solvent as reference. Glycerol solutions were warmed slightly (40 "C) prior to analysis to facilitate filling the 1-cm quartz cuvettes without entrapment of air. No color changes were apparent upon heating, but solutions were allowed to cool to room temperature prior to analysis. Reagents. m-Phenylenebis(trimethy1ammoniumiodide) (diquat, I) was synthesized according to the procedure described in ref 24. The reaction product was recrystallized once from warm 20230 (v/v) water-methanol. A small amount of diethyl ether was added to promote recrystallization. Phenyltrimethylammonium iodide (PTMA, 11) was synthesized from NJV-dimethylaniline and methyl iodide according to the procedure outlined in ref 25. Trimethylammonium chloride (reagent grade) was used as received from Eastman Chemicals. R~(bpy)~C1~.6H (bpy ~ O = 2,2'-bipyridine) was obtained from Strem Chemicals, Inc. The corresponding perchlorate salt was prepared by precipitating the complex from aqueous solution using HClO,. R ~ ( b p y ) ~ ( C 1 0 , ) ~ . 3 was H ~ Oprepared from the Ru(I1) salt by a procedure adapted from that for the oxidation of Fe"(bpy), complexes (26). The best yield was obtained as follows: R~(bpy)~C1,.6H~O (1.0 g) was dissolved in 40 mL of 0.5 M HZSO,. PbO, (1.14 g) was then added to oxidize the ruthenium complex (the reaction mixture changed from orange-red to green). After being allowed to stand for 1h, the solution was gravity filtered to remove unreaded PbOz and then cooled in an ice bath. HC104 (3.5 M, 6.0 mL) was added, causing formation of a green precipitate. The crystals were collected by vacuum filtration. Oxidation was confirmed by aqueous visible spectroscopy (see Results and Discussion). It was found that the addition of larger quantities of HC104 prevented precipitation of the product, probably due to protonation of the ligands and dissociation of the complex. Cu(bpy),(ClO,), was prepared by dissolving 0.37 g of CuCl,.HzO and 1.0 g of 2,2'-bipyridine in distilled water. The solution was subsequently treated with 5.0 mL of perchloric acid (60%) causing formation of a blue precipitate. The solution was cooled in ice and filtered. The isolated product was then recrystallized from hot water. D20 (99.8 atom % D; Sigma) was used as received for NMR. Glycerol was obtained from Fisher (reagent grade), Sigma (Sigma grade), or Aldrich (Gold Label). Except as noted above, all salts were reagent grade. Supporting electrolyte
715
+
MI2
Figure 1. EH mass spectrum of diquat I (0.14 mol YO)in glycerol with NaN03 supporting electrolyte (5.2mol % ) and KNO, internal standard (0.6 mol %): PTMA, phenyltrimethylammonium ion: TMA, trimethylammonium ion; M NO3, ion paired diquat. Intensities are relative to the [K G,]' peak at m l z 223. Sodium adducts are deleted for clarity.
+
+
(NaN03, NaC1, or NaI) and internal standard (KN03,KCl, or KI) were added to mass spectral samples to maintain the total ionic strength between 5.0 and 6.0 mol 9'0 (glycerol = 100 mol %). RESULTS AND DISCUSSION Reduction of rn -Phenylenebis(trimethylammoniun Iodide) (I). Figure 1 shows the mass spectrum of a solution of the diquat (I) in NaNOs/glycerol. The only species attributable to the unreacted diquat is the ion-paired adduct ([M NO3]+)at rn/z 256. Much more abundant is the PTMA ion (11) a t m / t 136 (PTMA:diquat = 5:l). Solvated PTMA at mJz 228 is also observed, but with low intensity and overlapping with a conventional metastable ion a t 228.6 (corresponding to the process [Na + G4]+ [Na + G3]++ GI). In an earlier study (15), proton NMR spectra of DzO solutions of the diquat, PTMA, and trimethylammonium (TMA) chloride established that the lighter ions in Figure 1 did not result from contamination of the diquat. However, attempts to confirm by NMR that PTMA was formed by reduction of the diquat by glycerol were unsuccessful because of broadening due to the high viscosity of glycerol solutions. Thus, it could not be confidently concluded whether reduction resulted from glycerol solution chemistry or from a process specifically associated with E H sampling. An NMR spectrum has now been obtained by preparing a solution of the diquat in glycerol and then diluting with DzO. This spectrum (Figure 2a) is characterized by peaks attributable to water (4.82 ppm), to glycerol (3.60 and 3.62 ppm), and to the diquat (methyl protons at 3.75 ppm and an aromatic multiplet from 8 to 8.4 ppm). There is a low-intensity signal near 3 ppm, possibly due to the methyl protons of the TMA ion. The low intensity of this signal indicates that reduction was not extensive, if it occurred at all. Consequently, the experiment was repeated, this time allowing the diquat to react in glycerol for 1 2 h prior to dilution in DzO (more closely approximating the degassing treatment used in the EHMS experiment). The resulting spectrum (Figure 2b) shows a resonance of significant intensity at 2.98 ppm, indicative of the TMA reduction product. A resonance at 3.72 ppm corresponds to the methyl protons of PTMA, although overlap with peaks due to glycerol complicates this region of the spectrum. These results suggest that the reduction does in fact occur in solution and that it is kinetically slow.
+
-
718
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
I 60
" _ ,
-
I
3
30
60
90
I20 150 TIME (hrs)
180
210
240
,l
720
+
Figure 3. Time dependence of the ratio [PTMA]+:[M NO3]+ (see Figure 1 for assignments). Each point represents an average from spectra obtained over a period indicated by the corresponding horizontal error bar. Vertical error bars represent standard deviations of the mean. The inset shows ratios from individual spectra for the second point in the plot (near 2 h) (0.14 mol % diquat, 5.2 mol % NaNO,, 0.60 mol % KN03).
Figure 2. (a) Proton NMR spectrum of diquat I (27.7 mg) dissolved in glycerol (123.3 mg) and then diluted with D,O (1.0 g) after 10 min (DSSinternal reference). (b) Proton NMR spectrum of diquat I (26.2 mg) dissolved in glycerol (104.3 mg) and then diluted with D,O (1.0 g) after 12 h (DSS internal reference).
Given this time scale, it should be possible to monitor the progress of the reaction in the mass spectrometer. To test this possibility, a glycerol solution of NaNO, (supporting electrolyte) and KN03 (internal standard) was degassed prior to the addition of the diquat. Mass spectra were then obtained over a 72-h period. Figure 3 shows the variation of the PTMA/diquat ratio with time. The first 30 min of the reaction could not be studied due to the time necessary to prepare and load the sample. However, the results a t subsequent times show that the relative abundance of PTMA increased over the first 3 h of the reaction and then leveled off, consistent with reduction in solution. Each point in Figure 3 represents an average of data from several individual mass spectra. The error bar in the x direction indicates the period over which the data set was obtained; the average ratio for each set is plotted at the average time. For each set, the emitter power supply was set to 4 kV and the solution was advanced to the needle tip to initiate emission. The emitter potential was then increased to 8 kV, and several spectra were obtained. Finally, emission was interrupted by shutting off the ion source power supplies. The PTMA/diquat ratio exhibited a systematic decrease among the spectral scans averaged for each point in Figure 3. For example, the inset of Figure 3 shows the data from individual scans in the second set, obtained after the reaction had proceeded for 2 h. The short-term decreases in the PTMA/diquat ratio evident in comparing successive spectra within a set, contrasted with the long-term increase in the set averages, indicate that a second process is active, probably the depletion of PTMA ions in the emission region due to a
higher sampling rate relative to the diquat (21). Figures 1 and 3 suggest that PTMA is present at a higher concentration than the diquat after even a short reaction time (assuming that sampling efficiencies are equal). By contrast, the integrated peak areas for the methyl protons at 3.75 and 2.98 ppm in the NMR spectrum of Figure 2b show a diquat/TMA ratio of roughly 25:1, even after 12 h of reaction. Taking into account the fact that there are two quaternary ammonium groups in the diquat, the reactant-to-product ratio is about 12:1, indicating only 8% conversion. The apparent disagreement with the mass spectral data could have arisen from two sources. First, the glycerol/diquat ratio was lower in solutions prepared for NMR. This was necessary both to reduce spectral interference from glycerol and to maintain low viscosity after dilution. The low glycerol concentration could have reduced the extent of reaction. Alternatively, the assumption of equal mass spectral sampling efficiencies could have been in error. To test for the second possibility, the sampling efficiency ( S )of PTMA was obtained by using a KNO, internal standard (with NaN03 supporting electrolyte)
S = UM/CM)/UK/CK)
(1)
where I M and CM are the signal intensity and concentration for PTMA, IKis the sum of signal intensities for [K Gz]+ and [K G3]+,and CK is the concentration of KN03. Only the PTMA peak at m / z 136 was used in the intensity calculation; as noted above, the solvated adduct a t 228 overlaps with a metastable ion at 228.6. By use of this sampling efficiency (70.3) and eq 1, the concentration of PTMA in the diquat solution of Figure 1 can be calculated from the known CK and the measured Zhl/ZK ratio (0.71). The value thus calculated is 0.006 mol % . On the basis of an initial diquat concentration of 0.135 mol %, this represents only 5% reduction, in good agreement with the NMR data (8% conversion). These calculations indicate that there is a significant sampling bias against the diquat in EHMS (compare the PTMA:diquat concentration ratio of about 1:19 with the roughly 5:l ion intensity ratio from Figure 1). A probable cause is the strong solvent-solute interactions between the doubly charged diquat and glycerol (191,evident in a spectrum obtained with the ESA voltage set to pass ions with roughly 75% of the acceleration voltage (Figure 4). In this spectrum, the doubly charged diquat is observed with up to nine glycerol
+
+
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988 0
0
X-
XJ
717
l K * GZ?
223
0
>g t
I
v,
a. 1 I'o
W
2%
M*G21
w 0
0
150
0
R
0
ZOO
M/2
pi
W
n W
'1
(TUA+G,r
152 ITMA). ,6,0
,
(Mar 321
0
0
50
1
02io
Figure 4. EH mass spectrum of diquat I analogous to that of Figure 1 except that the ESA voltage was reduced to pass singly charged ions with 6.0 keV of energy (V,,, = 8.0 kV).
$1
I
315
(L
-!-
0
IKIGS).
I
I
100
Flgure 5. EH mass spectrum of diquat I (0.12 mol %) in glycerol with NaI supporting electrdyte (5.1 mol %) and KNO,, internal standard (1.2 mol %). Labeling as in Figure 1, except that [M i- I]+ = ion paired diquat.
molecules attached, and the ion paired diquat is also detected with up to three glycerols. The absence of these ions in the spectrum of full energy ions (Figure 1) accounts a t least in part for the low diquat sensitivity. Ion pairing may also contribute to this low sensitivity, if ion charge is neutralized. As a test, a sample analogous to that of Figures 1 and 3 was prepared, using NaI instead of NaN03 for supporting electrolyte (Figure 5). The underlying assumption here is that ion pairing should vary with counterion. For the sample with I-, the [PTMA]/[diquat + I]+ ratio (roughly 13:l) was higher than for any of the nitratecontaining samples, and no long-term time dependence was observed. The high PTMA/diquat ratio may result from an ion-pairing induced change in the relative sampling efficiencies of PTMA and the diquat or from an increase in the extent of reduction. Indeed, the sampling efficiency of PTMA in a NaI/KI solution (37.4) was lower than that for the glycerol/NaN03 solution (70.3), reflecting the greater tendency for I- to form ion pairs with quaternary ammonium salts (27). This was more than offset by an increase in the concentration of PTMA in the diquat/NaI solution (estimated by using this sampling efficiency),to 0.04 mol 70,representing roughly 33% reduction. Corroboration of this ratio by NMR (as above) was not feasible, mainly because sufficient diquat and excess NaI could not be dissolved in a small amount of glycerol. However, it seems reasonable that an excess of I-, a reducing agent, might induce greater reduction of the diquat. Acceleration of the reaction by I- was also evident; equilibrium is apparently reached within the 30-min period inaccessible to MS measurements. The shift in equilibrium
1
I 2i5
11111 2io
245
260 zj5 MI2
zb
I
s b ~ 3io
J 3;5
Figure 6. EH mass spectrum of 0.01 mol ?LO R~(bpy),(C10,)~~6H,O, 5.0 mol % NaNO,, and 0.1 mol % K N 0 3 in glycerol. confirms that I- does not serve only as a catalyst; it must be a reactant. The fact that I- does not induce diquat reduction in aqueous solution indicates a synergism with glycerol. Other details of the mechanism are not known. It is interesting to note that extension of the calculations above to estimate sampling efficiencies for the diquat shows an increase of about 30% (from 1.0 to 1.3) on changing the principal anion from NO3- to I-. This is consistent with the data of Figure 4,wherein it was established that failure to detect doubly charged diquat ions contributed to the high PTMA.diquat ratio of Figure 1. Increased ion pairing induced by I- would reduce the fraction of diquat in the relatively poorly sampled 2+ ionic form, thereby increasing S. Reduction of Transition Metals and Complexes. In an earlier study of sampling efficiency (16), a doubly charged tris(bipyridy1) complex of Ru(1I) (Ru(bpy)32f)was sampled from glycerol with high sensitivity, attributed to increased interactions with the extracting field (relative to singly charged ions) and reduced ion-solvent interactions (due to ligand shielding of the metal ion charge center). This result contrasts sharply with the low S of the diquat, indicating that the diquat is subject to strong solvation at two distinct unshielded charge sites. In an attempt to assess the effect of charge on sampling efficiency independent of ion structure, the Ru(II1)-tris(bipyridyl) complex was synthesized and characterized by EHMS (with NaN0, supporting electrolyte) as part of the present study. The resultant mass spectrum (Figure 6) was identical with that for the Ru(I1) complex (16);no ions containing the trivalent metal ion were detected. This prevented assessment of the effect of charge on sampling, but provided instead a second test for reduction by glycerol. Based on the sampling efficiency of the Ru(I1) complex and a (evaluated with a solution containing R~(bpy),(ClO,)~ KNO, internal standard, using eq l),it is evident from Figure 6 that the reduction of Ru(II1) was virtually complete (Le., the intensity of the Ru(I1) complex is sufficient to account for all added Ru). This was true with all counterions tested (NO3-, C1-, and I-). Because the Ru(1I) and Ru(II1) complexes have distinct UV-vis spectroscopies, reduction can be confirmed by absorption spectroscopy. Figure 7a shows the UV-vis spectra of R~(bpy),(C10,)~*3H~O and R~(bpy)~C1,.6H,Oin 0.5 M aqueous H2S04 (the acid prevents hydroxide-induced reduction (26,28) while maintaining constant ionic strength). Comparison with previous reports (29,30) for these complexes confirms the presence of the Ru(I1) and Ru(II1) complexes. By contrast, the UV-vis spectrum for the Ru(II1) complex in glycerol is virtually identical with that for the Ru(I1) complex (Figure 7b), indicating reduction of the trivalent complex. (Small differences in absorptivity may be due to differences
718
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
a
Table I. S t a n d a r d Reduction Potentials (vs NHE) for Various Metal Ions a n d Complexes i n Water a n d Acetonitrile
l
E”, V in indicated solvent (ref) redox couple Ru(bpy),3+12+ CU+’O Co(b ~ y ) ~?+ ~ + / CU?+/0 CU2+I+
water 1.30 (32) 0.52 (34) 0.37 (32) 0.34 (34) 0.16 (34) 0.12 (32)
Cu (bpy)22+/ Cr(b~y)~~+/?+ Ni2+’0 -0.23 (34) Cr(b~y)~,+’+ Co(bpy)3z+I+ Ru(bpy)32+/+ +
y
acetonitrile
08
1.44 (33) 0.19 (35) 0.52 (33) 1.20 (35) -0.02 (33) -0.09 (36) -0.52 (33) -0.73 (33) -1.28“
0.0
i
c
450
500
I
I
400 WAVELENGTH ( n d
350
‘Estimated from the value for R ~ ( b p y ) ~ ~ + in/ acetonitrile ,+ (33) and the separation between the 3+/2+ and 2+/+ couples in acetonitrile (37).
in reagent purity, since the Ru(II1) complex was synthesized from the Ru(I1) complex.) The reduction is rapid, as the characteristic green color of the Ru(II1) complex can be observed to disappear as the complex (or a small volume of its aqueous solution) dissolves in glycerol. The reduction occurs even if a strong acid (p-toluenesulfonic acid) is added to the glycerol prior to dissolution of the complex. This indicates that the reaction is due to the reducing properties of glycerol, since it does not occur under similar conditions in aqueous solution. The spectroscopic changes cannot be attributed to solvatochromism; if an aliquot of a 0.8 mM solution of the Ru(II1) complex in glycerol is diluted with water (final conM), the orange color of Ru(bpy),2+ centration 1.5 X persists. In iight of this evidence of the importance of glycerol redox chemistry, a quantitative measure of the solvent’s “reducing hereafter denoted EoG)would clearly power” (i.e., EoGlycerol, be useful. However, oxidation of alcohols involves multiple electron transfer and substantial molecular rearrangement; this electrochemistry therefore tends to be irreversible and not suited to direct measurements of E” (31). This was confirmed for glycerol by attempting cyclic voltammetry of the neat solvent and its acetonitrile solutions (with NaClO, supporting electrolyte and R ~ ( b p y )internal ~ ~ + reference). In + / ~ +generates the absence of glycerol, the R ~ ( b p y ) ~ ~couple a reversible voltammogram ( E l l 2= (Epa+ E,C)/2 = 1.15 V vs SCE). Addition of glycerol induces two major changes in the appearance of the voltammogram. A highly irreversible anodic wave appears a t potentials more positive than the Ru( b ~ y ) ~ ~couple, + ” + presumably attributable to glycerol oxidation. More importantly, the symmetry of the Ru(bpy)33+/2+ wave is lost, with the anodic current increasing while the cathodic signal is attenuated. The detailed appearance of the voltammogram depends on several factors; e x p e r i m e n t s are
0.24
I , 530
400
45 0
WAVELENGTH
350
lnml
Figure 7. Visible spectrum of R~(bpy)~Cl,.3H,O (A) or Ru(bpy),(CI0,),.3H20 (B) in (a) 0.5 M aqueous H,SO,, [A] = 4.81 X loT5M, (B] = 7.46X M, and (b) glycerol, [A] = 1.20 X M, [B] = 1.33 x 10-4 M.
planned to elucidate details of the kinetics by product identification and careful consideration of voltammogram dependences on parameters including sweep rate, temperature, and concentrations. However, the conclusion of importance to the present study is already clear; the electrochemical data confirm the electrocatalysis of glycerol oxidation in acetonitrile by Ru(III), thereby again confirming the solution chemical reduction of R ~ ( b p y ) , ~by+ glycerol. This suggests that, while direct measurement is not feasible, EoGcan still be estimated by bracketing. Rough bounds can be established based on the fact that no Ru(bpy),+ was observed in these studies. The fact that glycerol is incapable of reducing the Ru(I1) complex indicates that EoG must lie
T a b l e 11. EH Mass S p e c t r a of Copper Salts i n Glycerol re1 intens of ion with indicated nusb sample Cu(NOJzC (3.7 mol 9’0) CUC1, (4.8 mol % )
detected ion”
n = 2
3
4
5
[Cu + G J 2 + [Cu + G n ] + [ C U N O+ ~ G,]’
40.9 30.5
3.6
96.9
11.9 10.5 100
[CuCI + G,]+
22.4
86.4
100
8.2
6 40.8
7
8
21.3
4.9
’G, indicates solvation by n glycerol molecules. Intensity of the most abundant ion in each isotope cluster. also detected as a solid precipitate when the sample was allowed to stand for some time.
For this sample, Cuo was
ANALYTICAL CHEMISTRY, VOL. 60, NO. 7, APRIL 1, 1988
Table 111. EH Mass Spectra of Cu(bpy)z(C104)z in Glycerol with NO, or C1m/z
sample
detected iona
Cu(bpy),(ClO,), (0.04 mol
[Cu(bpy),NO,]+ [Cu(b~~)zl+ [K + G3]+ [K + Gz]+
437 375 315 223
61.9 46.4 18.4 100
[Cu(bpy),Cl]+ [Cu(b~~)zl+ [K + G3]+ [K + G2]+ [C~(bpy),]~+
410 375 315 223 187.5
114.8 17.6 50.1 100 44.9
+ NaN0,
(5.40 mol %)
+ KNO, (0.20 mol % )
Cu(bpy)z(CIO,)z(0.03 mol %)
+ NaCl (4.50 mol % ) + KCl (0.10 mol %)
G denotes solvating glycerol. ion in each isotope cluster.
re1 intensb
Intensity of the most abundant
between E" for R ~ ( b p y ) , ~ +and / ~ +Ru(bpy)gP+/+. Values for these Eos in glycerol are not available and cannot be readily attained due to the high viscosity of this solvent. Comparison of values for aqueous and acetonitrile (AN) solutions (Table I) shows a significant solvent effect, which, however, is relatively smaller for complexes than for simple solvated ions. Behavior in glycerol (a protic solvent) should more closely resemble aqueous chemistry. Thus, as a first approximation it can be concluded that E"G must lie between 1.44 V (E" for R~(bpy)~,+/,+ in AN) and -1.28 V (E" for Ru(bpy)?+/+ in AN) and is probably less than 1.30 V (E" for R~(bpy),,+/~+ in water). This broad range can be narrowed by considering the studies of Segur (%), who reports that warm glycerol can effect reduction of Cuz+to Cuo (except in the presence of C1- or other ligands which stabilize the higher oxidation state). Consistent with this, reduction of Cu2+ was evident in the E H mass spectrum of Cu(NO,), in glycerol, but not that of CuC1, (Table 11). EoGmust therefore be less than 0.34 V. In fact, detection of solvated Cu(1) in the spectrum of the Cu(N03), solution confirmed by deindicates that E"G C 0.16 V (Eocu(II)lcu(I)), tection of Cu(bpy),+ in the EH mass spectrum of a solution of Cu(bpy),(C104), (Table 111). Chloride counterion impedes (but does not eliminate) this reduction, as it did for the simple CuCl, salt. The observation of Cu in two oxidation states in the Cu(bpy), system (in contrast with the dominance of Ru( b p ~ ) ~indicates ~+) that E", must in fact lie rather close to E" for reduction of the Cu complex (0.12V). This is consistent with the fact that Co(bpy),3+ was reduced to C ~ ( b p y ) , ~but +, not to Co(bpy)3+ (Table I), and similarly that the corresponding Cr(II1) complex was only partially reduced to Cr(I1) (16). Finally, absence of reduction in the E H mass spectrum of Ni(N03), (39) (a transition metal of lower E"; Table I) indicates a lower limit of -0.23 V for EOG, consistent with the conclusion that E" is near 0.1 V. CONCLUSIONS These results clearly indicate that the solvent can contribute directly to reduction, independent of MS ionization. While not excluding bombardment-induced reduction in FAB (e.g., the purple color indicative of reduction of paraquat becomes evident only after bombardment (7)),this shows that alternative mechanisms should be considered. The results of this study also show the importance of considering sampling efficiencies when studying solution processes by EHMS. The extent of reduction of the diquat
719
could not be accurately assessed from the mass spectrum until the sampling efficiency of the product was determined. Further assessment of the factors that control sampling efficiency in EHMS may ultimately improve its utility for quantitation of solution chemistry without the need for standards. ACKNOWLEDGMENT Helpful discussions with J. Q. Chambers of the University of Tennessee are gratefully acknowledged. LITERATURE CITED (1) Cook, K. D. Mass Spectrom. Rev. 1988, 5 , 467-519. (2) Barber, M.; Bordoli, R. S.; Segwick, R. D.; Tyler, A. N. J . Chem. Soc., Chem. Commun. 1981, 325-327. (3) Yamashita, M.; Fenn, J. B. J . Phys. Chem. 1984, 8 8 , 4451-4459. (4) Vestal, M. L. Science 1984, 226, 275-281. (5) Thomson, B. A.; Iribarne, J. V.; Dziedzic, P. J. Anal. Chem. 1982, 54. 2219-2224. (6) Pelzer, G.; DePauw, E.; Dung, D. V.;Marien. J. J . Phys. Chem. 1984, 88,5065-5068. (7) Clayton, E.; Wakefield, A. J. C. J . Chem. Soc., Chem. Commun. 1984, 969-970. (8) Cerny, R. L.; Bursey, M. M.; Jameson, D. L.; Malachowski, M. R.; Sorrell, T. N. Inorg. Chlm. Acta 1984, 8 9 , 89-93. (9) Javanaud, C.; Eagles, J. Urg. Mass Spectrom. 1983, 78, 93-98. (10) Bojesen, G. Urg. Mass Spectrom. 1985, 2 0 , 413-415. (11) Williams, D. H.; Findeis, A. F.; Naylor, S.;Gibson, B. W. J . Am. Chem. SOC. 1987, 109, 1980-1986. (12) Gale, P. J.; Bentz, B. L.; Chait, B. T.; Field, F. H.; Cotter, R . J. Anal. Chem. 1988, 58, 1070-1076. (13) Cerny, R. L.; Gross, M. L. Anal. Chem. 1985, 57, 1160-1163. (14) Field, F. H. J . Phys. Chem. 1982, 8 6 , 5115-5123. (15) Cook, K. D.; Chan, K. W. S. Int. J . Mass Specfrom. Ion Processes 1983, 54, 135-139. (16) Chan, K. W. S.;Cook, K. D. J . Am. Chem. SOC. 1982, 704, 503 1-5034. (17) Man, V. F.; Lin, J. D.; Cook, K. D. J . Am. Chem. SOC. 1985, 707. 4635-4640. (18) Lai, S.-T. F.; Chan, K. W. S.; Cook, K. D. Macromolecules 1980, 73, 953-956. (19) Chan. K. W. S.;Cook, K. D. Anal. Chem. 1983, 55, 1306-1309. (20) Stimpson, B. P.; Simons, D. S.;Evans, C. A. J . Phys. Chem. 1978, 8 7 , 660-670. (21) Callahan. J. H.; Hool, K.; Reynolds, J. D.; Cook, K. D. Int. J . Mass Spectrom. Ion Processes 1987, 75,291-317. (22) Murawski, S. L.; Cook, K. D. Anal. Chem. 1984, 5 6 , 1015-1020. (23) Stimpson, B. P.; Evans, C. A. J . Electrost. 1978, 5 ,411-430. (24) Torf, S. F.; Khronov-Borisov, N. V. J . a n . Chem. USSR (fngl. Trans/.) 1980, 3 0 , 1782-1787. (25) Vogel, A. I. Practlcal Organic Chembfry; 3rd ed.; Wiley: New York, 1986; p 660. (26) Nord, G.; Weinberg, 0. J . Chem. Soc., Dalton Trans. 1972, 866-868. (27) Evans, D. F.; Matesich, M. A. J . Solution Chem. 1973, 2 , 193-216. (28) Brandt, W. W.; Dwyer, F. P.; Gyarfas, E. C. Chem. Rev. 1954, 54, 959-101 ... . .. 7 . .. (29) Heath, G. A.; Yellowlees, L. J.; Bratesman, P. S. J . Chem. SOC., Chem. Common. 1981. 287-289. (30) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 3rd ed.; Interscience: New York, 1972; pp 1000-1017. (31) Sundholm, G. J . Nectroanal. Chem. 1971, 3 7 , 265-267. (32) McWhinnie, W. R.; Miller, J. D. Adv. Inorg. Chem. Radiochem. 1989, 12, 145-146. (33) Saji, T.; Aoyagui, S. J . flectroanal. Chem. 1975, 6 0 , 1-10, (34) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980; pp 699-701. (35) Kolthoff, I . M.; Coetzee, J. F. J . A m . Chem. SOC. 1957, 7 9 , 1852-1858. (36) Saji, T.; Aoyagui, S. J . Electroanal. Chem. 1975, 56,401-410. (37) Tokel, N. E.; Bard, A. J. J . Am. Chem. SOC. 1972, 9 4 , 2862-2863. (38) Segur, J. B. In Glycerol; Miner, C. S., Dalton, N. N., Eds.; Reinhold: New York. 1953; pp 335-396. (39) Cook, K. D.; Callahan, J. H.; Man, V. F. Anal. Chem. 1988, 60 706-7 13.
RECEIVEDfor review July 29,1987. Accepted December 17, 1987. This research was supported by the National Science Foundation Division of Materials Research (Grant DMR8406825, jointly funded by the U.S. Army Research Office).
